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This Week in Science

Double Helix, Doubled

Chromatin consists of genomic DNA packaged onto nucleosomes—double donut-shaped complexes of histone proteins. Roughly 150 base pairs of DNA are wrapped around each nucleosome with variable lengths of linker DNA in-between. Using cryogenic electron microscopy, Song et al. (p. 376; see the Perspective by Travers) determined the 11 angstrom–resolution structure of a 12-nucleosome string of DNA. The segment forms a 30-nanometer fiber structure, which is itself double-helical, like the DNA it packages.

Africa's Bane

Tsetse are blood-feeding, fast-flying flies that transmit a range of Trypanosoma spp. protozoan pathogens, which cause sleeping sickness in humans and their nagana in their livestock. The International Glossina Genome Initiative (p. 380) sequenced the genome of Glossina morsitans and identified the genes for many attributes of the tsetse's remarkable biology, including viviparity and the expression of analogs of mammalian milk proteins. Tsetse are host to several specific symbionts that appear to synthesize essential nutrients for the fly and also to hitherto undiscovered parasitoid-derived viruses. Deeper exploration of this genome will reveal what makes these fly species so host- and trypanosome specific.

Understanding N Loss

Biologically available nitrogen (N) is essential for marine plants, and shortage of N limits photosynthesis. Marine N can be removed by denitrification and anaerobic ammonia oxidation (anammox) processes, but what controls the balance between these two pathways? Babbin et al. (p. 406, published online 10 April) tested the effects of stoichiometry on N removal in the lab and found that the balance of N loss processes depends on the stoichiometry of the source organic material.

Optogenetic Insights

Mapping functional neural circuits for many behaviors has been almost impossible, so Vogelstein et al. (p. 386, published online 27 March; see the Perspective by O'Leary and Marder) developed a broadly applicable optogenetic method for neuron-behavior mapping and used it to phenotype larval Drosophila and thus developed a reference atlas. As optogenetic experiments become routine in certain fields of neuroscience research, creating even more specialized tools is imperative (see the Perspective by Hayashi). By engineering channelrhodopsin, Wietek et al. (p. 409, published online 27 March) and Berndt et al. (p. 420) created two different light-gated anion channels to block action potential generation during synaptic stimulation or depolarizing current injections. These new tools not only improve understanding of channelrhodopsins but also provide a way to silence cells.

A Dual Approach to 2 + 2

Asymmetric catalysis generally accelerates the pathway to one specific product geometry that can be manipulated by reducing the temperature to slow down competing reactions. It is more difficult to be selective in photochemical reactions, but in the [2 + 2] coupling of olefins to make four-membered rings, Du et al. (p. 392; see the Perspective by Neier) used a ruthenium catalyst that absorbs visible light to activate the substrates below the frequency threshold where they absorb intrinsically. Then a second—a chiral Lewis acid—catalyst directs the product stereochemistry. A major advantage of the dual reactions is that each catalyst can be tuned independently.

Wetted Apatite

The long-running story of the dry Moon was rewritten a few years ago when hydrogen-bearing glass spherules were discovered. The highest water contents are found in lunar apatite, at levels suspiciously comparable to the water content of Earth apatites. Boyce et al. (p. 400, published online 20 March; see the Perspective by Anand) now show that the water content of lunar apatite is not a reliable indicator of the abundance of water in mare basalts. The existence of apatite with high water content is an almost inevitable consequence of the loss of tiny amounts of fluorine-rich apatite from a melt and replacement by hydrogen and is thus no indication of a “wet” Moon.

House or Face?

The neural mechanisms of spatial attention are well known, unlike nonspatial attention. Baldauf and Desimone (p. 424, published online 10 April) combined several technologies to identify a fronto-temporal network in humans that mediates nonspatial object-based attention. There is a clear top-down directionality of these oscillatory interactions, establishing the inferior-frontal cortex as a key source of nonspatial attentional inputs to the inferior-temporal cortex. Surprisingly, the mechanisms for nonspatial attention are strikingly parallel to the mechanisms of spatial attention.

No Light Control

Light is the main source of energy for plants and is also used as a signal for growth and development: Indeed, it can modulate up to a fifth of the entire transcriptome in both Arabidopsis thaliana and rice. Petrillo et al. (p. 427, published online 10 April) show that light can affect gene expression through alternative splicing of the serine-arginine rich protein At-RS31, required for proper plant growth. But photoreceptors are not involved; rather, a mobile retrograde signal from the chloroplast controls the alternative splicing of At-RS31.

Magnified Flare-Up

The rise and fall of the luminosity of a supernova detected in 2010 was typical for its class, but its apparent brightness was 30 times greater than similar events. Quimby et al. (p. 396) compared spectra from the time of peak brightness and after the supernova faded, from which they concluded that something was interfering with our line of sight to the supernova. A previously unknown foreground galaxy turned out to be acting as a lens, bending and magnifying the light from the supernova. Potentially, spacetime warping like this could allow direct testing of cosmic expansion.

Deep Freeze

Geologists usually consider glaciers and ice sheets to be gigantic abrasives, scouring the ground beneath them and carving out relief on the underlying landscapes. Bierman et al. (p. 402, published online 17 April) show that this is not always the case. They found that the silt at the very bottom of the Greenland Ice Sheet Project 2 core contained significant amounts of beryllium-10, an isotope produced in the atmosphere by cosmic rays and which adheres to soils when it is deposited on them. Hence, the dust at the bottom of the ice sheet indicates the persistence of a landscape under 3000 meters of glacial ice that is millions of years old.

RNA Heteroplasmy

Like nuclear DNA, the mitochondrial genome has to be posttranscriptionally modified to function properly; however, among individuals, mitochondrial RNA (mtRNA) transcripts vary in ways that are poorly understood. Hodgkinson et al. (p. 413) looked at mtRNA editing events and posttranscriptional methylation in more than 700 individuals. Interestingly, variation at the ninth position within transfer RNAs showed a high frequency of variation that, in some cases, is genetically attributable.

Cyanobacterial Diversity

What does it mean to be a global species? The marine cyanobacterium Prochlorococcus is ubiquitous and, arguably, the most abundant and productive of all living organisms. Although to our eyes the seas look uniform, to a bacterium the ocean's bulk is a plethora of microhabitats, and by large-scale single-cell genomic analysis of uncultured cells, Kashtan et al. (p. 416; see the Perspective by Bowler and Scanlan) reveal that Prochlorococcus has diversified to match. This “species” constitutes a mass of subpopulations—each with million-year ancestry—that vary seasonally in abundance. The subpopulations in turn have clades nested within that show covariation between sets of core alleles and variable gene content, indicating flexibility of responses to rapid environmental changes. Large sets of coexisting populations could be a general feature of other free-living bacterial species living in highly mixed habitats.